This invention relates generally to optical signal receivers. This invention relates particularly to a method for minimizing polarization signal fading in an optical receiver included in a fiber optic interferometric sensor system. Still more particularly, this invention relates to a mask selection algorithm for a tri-cell polarization diversity detector that increases the probability of selecting the mask element with the largest signal for a given input.
Mismatched fiber optic interferometers are commonly used as sensing elements in fiber optic sensor arrays for measuring changes in a parameter such as fluid pressure, acceleration, magnetic field intensity, etc. Such sensing elements measure the time-varying phase delay between optical signals that have propagated along separate optical paths having unequal path length.
Mixing between a reference signal and a data signal is often necessary to extract information from an optical carrier. In interferometric sensing the mixing is typically between a reference signal and a signal whose phase has been modified, or modulated by the parameter being measured.
Modulation is commonly used to transmit information from an information source, such as a sensor system where information is detected, to an information destination, such as a receiver system where detected signals arc received and processed. According to conventional modulation techniques, a signal of interest detected by a sensor modulates a carrier signal by modifying one or more characteristics of the carrier signal, such as amplitude, frequency or phase, to form a modulated carrier signal. The modulated carrier signal is then more easily transmitted over the appropriate communication channels to the destination or receiver system where the modulated carrier signal is demodulated to recover the signal of interest and determine the information.
The fiber optic sensors detect or sense signals that modulate the output phase of the sensor system or interferometer. The modulated carrier can then transmitted to a receiver system and photodetected. In a system having an array of sensors, the signals are often multiplexed, for example, using time division multiplexing (TDM) and/or wavelength division multiplexing (WDM), as well as frequency division multiplexing (FDM).
Fiber optic sensor systems acquire in the demodulation process a signal component proportional to the sine of the sensor phase shift and another signal component proportional to the cosine of the sensor phase shift. The sine of the sensor phase shift is referred to as the quadrature term, Q; and the cosine of the sensor phase shift is referred to as the in-phase term, I. The angle of the phase shift is determined by calculating the ratio I/Q, which is the arctangent of the sensor phase shift. The amplitudes of the sine and cosine terms must be set equal by a normalization procedure to ensure the accurate implementation of an arctangent routine to determine the sensor phase shift.
One type of modulation technique implemented in interferometric and other sensing systems involves the use of phase generated carriers. The time varying phase signal (signal of interest) of each sensor modulates the phase generated carriers to form modulated carriers. Both the signals of interest and the phase generated carriers can be mathematically represented as a Bessel series of harmonically related terms. During modulation, the Bessel series of the signals of interest modulates the Bessel series of the phase generated carriers. The number of terms in the Bessel series of the resulting modulated carriers will be dependent upon the amplitude of the measured or detected signal of interest. The harmonically related terms in the Bessel series of the modulated carrier represent both the measured or detected signal of interest and the carrier signal.
Typical fiber optic sensor systems using phase generated carriers to transmit a detected or measured signal (signal of interest) to a receiver system have used a pair of quadrature carriers with frequencies of either ωc and 2ωc or 2ωc and 3ωc, where ωc is the phase generated carrier frequency. In multiplexed sensor systems, the sensor sampling frequency fs must be selected to ensure that frequencies greater than fs/2 are not aliased into the band of interest below fs/2.
In some systems the optical signal input to the interferometer is a phase generated carrier produced by generating time-dependent variations in the frequency of the optical signal output by a laser. A phase generated carrier may be produced by various techniques. One technique involves routing the laser source output through an external phase modulator and applying a sequence of separate and unique linear ramp voltages to the linear phase modulator to produce step changes in the optical frequency.
Another technique for producing a phase generated carrier uses sinusoidal phase modulation of the source signal. Instead of sampling signals associated with separate optical frequencies, the sampling of signals is associated with integration over portions of a period of the phase generated carrier.
Still another technique for producing a phase generated carrier involves the use of a Direct Digital Synthesizer (DDS) containing a numerically controlled oscillator (NCO). In particular, carriers that are 180° out of phase with the NCO phase will produce sensor responses with opposite sign after demodulation different than those produced by carriers that are in phase with the NCO phase in the DDS. When coherently combined, sensor responses with opposite signs will combine destructively, which results in an attenuation of the combined output and a reduction in overall system dynamic range.
A significant problem in systems that employ the reception of optical signals from an optical fiber is signal fading caused by changes in the polarization of the optical signals transmitted through the optical fiber. Specifically, phase information from two or more optical signals propagated through a fiber optic transmission line can be lost at the receiver if the polarizations of two signals of interest are crossed, resulting in no detector beat note. It is therefore necessary to provide some mechanism for treating the signal that yields a suitably large detector beat note for signal processing in all cases of polarization orientation.
Polarization diversity detectors are used to detect an optical signal of random time varying polarization and produce an electrical output corresponding to a selected polarization component in the optical signal. U.S. Pat. No. 5,852,507, which issued Dec. 22, 1998 to David B. Hall and which is assigned Litton Systems, Inc., assignee of the present invention, discloses a tri-cell polarization diversity detector that produces multiple output signals from an incident beam that has two orthogonal polarization components. The disclosure of U.S. Pat. No. 5,852,507 is incorporated by reference into the present disclosure.
U.S. Pat. No. 5,448,058, which issued Sep. 5, 1995 to Arab-Sadeghabadi and vonBierein and which is assigned Litton Systems, Inc., assignee of the present invention, discloses a polarization diversity detector that includes an array of three polarizers having axes of polarization spaced apart by selected angles such that an optical signal incident on the polarizer array has a component along at least one of the axes of polarization. A photodetector array is arranged such that each photodetector receives light from a selected one of the polarizers. At least one of the photodetectors receives parallel polarization components that form an electrical signal that indicates interference between the parallel polarization components. The disclosure of U.S. Pat. No. 5,448,058 is incorporated by reference into the present disclosure.